Polyamides, being excellent in heat resistance and gas barrier properties are used as packaging material for food, such as retort-packaged pouch food. The characteristics required of polyamide have recently been diversified with the steadily broadening application to food packaging. For example, there has been a demand for a polyamide providing a thin film having mechanical strength sufficient for practical use and excellent gas barrier properties, particularly a polyamide fit for productive successive biaxial stretching.
In general, on being stretched, a film prepared from a crystalline polymer reduces it thickness and increases mechanical strength per unit as is well known in the art. The stretching technique has been used to manufacture thin and mechanically strong films from various crystalline polymers.
However, it is recognized in the art that a film, a monofilament, a fiber, etc. of a crystalline polyamide, such as nylon 6 or nylon 66, tends to suffer from uneven stretching or tends to be broken at a relatively low draw ratio on being stretched unless the stretching conditions are controlled within narrow ranges. Where, in particular, a nylon film is subjected to successive biaxial stretching, which is deemed advantageous for mass production, by first-stage stretching (which may hereinafter be called "primary stretching") in the extruding direction of the film followed by second-stage stretching (which may hereinafter be called "secondary stretching") in the direction perpendicular to the extruding direction, it is known that the secondary stretching meets difficulty. This is considered to be because the polyamide molecules are oriented in parallel to the film surface by the primary stretching to form hydrogen bonds among molecules, whereby crystallization seems to proceed to make the film hard, which will make secondary stretching difficult. Therefore, it has been demanded to develop a polyanude which has excellent stretching properties fit for successive biaxial stretching.
A large number of proposals have been made to date as for improvement on stretching properties of polyamide. For example, JP-A-52-104565 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process for producing a successive biaxial stretching film, which comprises primary stretching a polyamide film to a planar orientation index of 0.6 to 1.5 and secondly stretching the film, wherein the film to be stretched is prepared by melt molding a polyblend of an aliphatic polyamide and a polyamide comprising xylylenediamine and an aliphatic .alpha., .omega.-dicarboxylic acid. JP-B-55-41901 (the term "JP-B" as used herein means an "examined Japanese patent publication") teaches a process for producing a successive biaxial stretching film which comprises primary stretching a film to a planar orientation index of 0.6 to 1.5 and secondly stretching the film, wherein the film to be stretched is prepared from a composition comprising an aliphatic polyamide, other polyamide that is compatible with the aliphatic polyamide, and a nucleating agent of such an amount that the isothermal crystallization parameter may become 1.1 to 2.5. JP-B-62-25704 proposes a process for producing a stretched film from a polyblend comprising an aliphatic polyamide and a polyamide comprising 2,2,4-trimethyl-1,6-diaminohexane and/or 2,4,4-trimethyl-1,6-diaminohexane and an aromatic and/or alicyclic dicarboxylic acid in a specific blending ratio. JP-B-6-43552 discloses a polyamide composition suited to biaxial stretching which is obtained from a polyblend of a copolyamide comprising an aliphatic polyamide and a semi-aromatic polyamide and a semi-aromatic polyamide comprising an aliphatic diamine and isophthalic acid and/or terephthalic acid.
Copolyamides having improved stretching properties that have been proposed include a copolyamide comprising .epsilon.-caprolactam, hexamethyleneisophthalamide and hexamethyleneterephthalamide (JP-A-53-5250) and a polyamide comprising at least a dicarboxylic acid and an alicyclic diamino compound (JP-A-60-104312).
JP-A-53-88053 offers a process for producing a successive biaxial stretching film from a polyblend comprising an aliphatic polyamide and a polyamide containing 50 mol % or more of a polyamide unit composed of 2,2,4- and/or 2,4,4-trimethylhexamethylenediamine and an aromatic and/or alicyclic dicarboxylic acid. JP-A-62-127346 discloses a polyamide composition comprising a copolyamide obtained from an aliphatic polyamide and a semi-aromatic polyamide and a semi-aromatic polyamide obtained from an aliphatic diamine and isophthalic acid and/or terephthalic acid.
The proposed copolyamides or polyamide compositions exhibit improved stretching properties as compared with a single polyamide, such as nylon 6, but the reached degree of improvement cannot be said to be sufficient. That is, the maximum draw ratio at which a film, etc. can be stretched uniformly is sometimes low. For example, uneven stretching tends to occur where the draw ratio exceeds 3. Further, when secondary stretching is carried out by successive biaxial stretching, they often suffer from uneven stretching or breaks at a low draw ratio.